1. Field of the Invention
The present invention relates to a waveguide-type optical splitter and a waveguide-type optical module having the same which are used in such as a PON (Passive Optical Network) system in order to branch off light into plural.
2. Description of the Related Art
Applications to subscribers of the PON system have been active for establishing optical communication systems at low cost. The PON system indicates the efficient use to be possible by branching off an optical fiber with an optical splitter in access sections, thus remarkable cost reduction is realized.
In this PON system, an optical splitter module is used in branching off of light. As this optical splitter module, required are a low insertion loss, low polarization dependence and high reliability. A PLC (Planar Lightwave Circuit) type optical splitter satisfies these needs and has been produced several tens of thousands per month only in Japan as a key device of the PON system.
In the PLC type optical splitter, while it has characteristics as compact and high reliability, double refraction is generated due to the difference in a linear expansion coefficient between a Si substrate and SiO2 thereby causing an adverse influence on optical characteristics. When a Y-branch is formed in a waveguide where the double refraction is generated, a branch ratio has a feature of polarization dependence thereby causing branched light to generate a PDL (Polarization Dependent Loss) which has a dependence on polarization. The PDL has an effect on the power margins of the optical communication systems, consequently, it is necessary to reduce of the same.
With regard to this PLC type optical splitter (a first conventional example), description will be provided in detail with reference to FIG. 1.
A PLC type optical splitter 80 is a configuration of 1×8 in which a waveguide 82 is formed on a substrate 81. An incident end 83 of the substrate 81 is connected to an optical fiber for input 84 and an output end 85 of the substrate 81 is connected to optical fibers for output 86. Then, the waveguide 82 is branched off into a plurality of waveguides by a Y-branched structure of a multi-stage. The plurality of waveguides which are branched off are connected to the optical fibers for output 86 respectively.
However, when the Y-branched structure of the multi-stage is applied to the PLC type optical splitter 80 in which the double refraction is large, the PDL cumulatively increases according to the number of the branch, therefore it may not meet the needs from customers. Particularly, even though it may not cause any problem in the current configurations of 1×4 and 1×8, the cumulated PDL cannot be neglected in multiple branch structures such as the one of 1×32. Further, an insertion loss will be increased by the occurrence of a branch excess loss at the Y branch in the PLC type optical splitter 80 with multiple branches.
In order to overcome the aforementioned problems of the PLC type optical splitter 80 and to realize favorable optical characteristics, employing so-called a “star coupler” which obtains multiple branches at single will be desired. However, in the conventional star couplers, despite optical power is large in a waveguide for output at the center, it becomes small in a waveguide for output at the end. Accordingly, the conventional star couplers have weakness in that loss uniformity in each port after being branched off is inferior to the one in the Y-branched type while the favorable polarization dependence can be obtained in the waveguide where the double refraction is large. The Patent Document 1 discloses a star coupler intended to solve such problems.
The star coupler of this Patent Document 1(a second conventional example) will be described in detail with reference to FIG. 2.
A star coupler 90 is so configured such that a waveguide for input 92, a sector slab waveguide 93, a plurality of tapered waveguides 94 and a plurality of waveguides for output 95 are sequentially arranged and connected on a silicon substrate 91. In the star coupler 90, the tapered waveguides 94 are respectively disposed in all of the waveguides for output 95. Also, by setting the opening width of the sector slab waveguide 93 side for each tapered waveguide 94 as narrow in the center with large light intensity and, on the contrary, as wide in the vicinity with small light intensity, all the intensity of signal lights which are branched off into the waveguides for output 95 are intended to be consistent (that is, the loss is to be uniformed).
Patent Document 1: Japanese Patent 2,538,099
Meanwhile, in the star coupler 90, an angle of diffraction of light propagating in the sector slab waveguide 93 has the dependence of wavelength. Consequently, for example, the loss uniformity between the waveguides for output 95 is to be altered in the wavelengths of 1.55 μm and 1.3 μm because the angles of diffraction are different. Hence, in the waveguides for output 95 in the vicinity, even though the light intensity is sufficient in some wavelengths, it is insufficient in other wavelengths. In this manner, the result is that the optical characteristics have dependence on the wavelengths.